Wireless communication systems including those based on direct sequence spread spectrum (DSSS) code division multiple access (CDMA) technology offer many benefits for cellular radio communications. In conventional CDMA receivers, known as single-user detectors (SUD), each user's data is estimated without consideration of the other users that are communicating simultaneously. The other users appear as background noise. These conventional receivers typically utilize simple correlation receivers that correlate the received signal with a synchronized copy of the desired user's spreading signal. An alternate approach is to employ a multi-user detector (MUD) that simultaneously demodulates all users within a CDMA bandwidth.
Consideration of the other users in detecting a particular user's signal can significantly improve the receiver's performance metrics. The improvement in performance of the MUD over the SUD is manifested either as a reduction in the required energy per bit (Eb) for a specified quality of service (QoS) for a fixed number of users, or as an increase in the number of users supported at the specified QoS of the same Eb. While the former offers the potential benefit of extending the lifetime of subscriber unit (mobile station) batteries and of reducing the overall interference in a CDMA cellular system, the latter represents a potential increase in the capacity of the system.
There are several design approaches for a MUD receiver. One approach is to remove from the received signal the estimated contribution of the other users, or what is referred to as the multiple-access interference (MAI). The estimated MAI may be entirely removed in a “brute-force” interference cancellation (IC) approach or only partially removed in so-called partial interference cancellation (PIC). The user's transmitted information is then estimated from the “cleaned” signal. Receivers that incorporate MAI reduction, or IC, are known as subtractive MUD. The performance of these receivers depends on the quality of the MAI estimates. The performance of these receivers also depends on the partial interference coefficients used to estimate the received signal. If the estimates are poor, the job of suppressing MAI may turn out to be ineffective. It is typical that hard estimates and fixed brute-force coefficients are used, which in some cases, may cause the MUD to perform worse than a conventional SUD.
Thus, there is a need for a method and apparatus for partial interference cancellation in a communication system, and particularly, for method and apparatus for enhancing the quality of the data estimates and cancellation coefficients utilized in providing partial interference cancellation.